Downhole running cables are used in the oil and gas industry for deploying and retrieving well intervention and logging equipment in a well. For example, tools can be deployed downhole using a slickline spooled out from a drum and guided over sheaves before entering the well. Steel wires are generally chosen for such services to meet the rigorous physical requirements of the service while maintaining tensile strength without sustaining damage. However, if the deployed tool relies on electrical signals, steel wires are not typically used to for communicating the electrical signals. Instead, copper conductors are used for this purpose. Since the copper cannot sustain load, the cable is reinforced with steel wire.
FIG. 1 illustrates a typical rig up system 10 for running cable 12 downhole for various purposes. The system 10 is shown for slickline, but the rig up for wireline (e-line), braided line, Heavy Duty Wireline Fishing (HDWF) line, and the like may use the same general configuration. The difference is that wireline (e-line), braided line, and HDWF line use a grease injection system to maintain well pressure. As shown, slickline instead uses a stuffing box 32.
The cable 12 (e.g., slickline, braided wireline, electric line, etc.) passes from a drum 22 in a deployment unit 20 to a hay pulley 28, which directs the cable 12 to the sheave on the stuffing box 32. The cable 12 enters the stuffing box 32, passes through a chemical injection sub 34, and a lubricator 36, and passes to a secondary barrier 38 or blow out preventer. Eventually, the cable 12 passes to the Christmas tree 40 through the swab and master valves 42, and then to the well for its intended purposes. Various other components are used with the system as well, but are not described here. When the cable 12 is used for intervention, for example, the rig up system 10 may include cable cutter subs, a tool trap, a tool catcher, check valves, etc.
The stuffing box 32 packs off around the cable 12. The chemical injection sub 34 applies various agents and corrosion inhibitors to the cable 12 during operations. The lubricator 36 is used for inserting and retrieving a tool string (not shown) when the well is under pressure. The secondary barrier 38 can use ram seals to close off around the cable 12 in the event of an emergency or essential maintenance.
For those cables 12 with a smooth outer surface, the stuffing box 32 can use elastomeric seals. Otherwise, grease-injected sealing hardware is used with served or braided cable surfaces. Where a stuffing box 32 cannot be used, for example, a grease injection control head (not shown) can create a seal around the moving cable 12 by injecting grease so the cable 12 can be run for intervention operations in wells under pressure.
The rig up's deployment unit 20 can be skid mounted on the rig or can be part of a deployment truck. The unit 20 stores the cable 12 on the drum 24 that feeds the cable 12 on and off of the unit 20. A winch for the drum 24 has a hydraulic drive powered by a diesel engine or electric power pack that drives the drum 24 to feed or pull the cable 12. The unit 20 may also include depth and tension systems. For example, a weight indicator sensor 29 can be used to measure line tension on the cable 12, and a depth counter 26 can be used to measure the length of cable 12.
As an example, FIG. 2 schematically shows a measuring device 50 that can measure depth and tension for the deployment unit (20). This measuring device 50 uses two wheels 52, 54 to measure depth of the cable 12. The device 50 mounts in front of the drum (24) on a spooling mechanism and can ride back and forth on linear bearings. The cable 12 from the drum (24) completely wraps around both wheels 52, 54 and extends from the wheels 52, 54 to other components to go to the well. Spooling rollers 55, guide rollers 56, and a pressure wheel 58 keep the cable 12 in the wheels 52, 54 and assist in spooling of the cable 12. Line tension is measured from a load pin axle 53 for the tension wheel 52. A hydraulic load cell may also be included that measures cable tension independently of the electronic load pin. Depth information is provided by an encoder 57 to the unit's control panel (22), which accounts for the size and stretch of the cable 12.
The cable 12 can come in various arrangements and geometries. Some forms of downhole running cables, such as wirelines, e-lines, braided lines, etc., have wires or strands. For example, FIG. 3A illustrates a cross-sectional view of a cable 60a according to the prior art. This cable 60a is a Dyform® Well Service strand available from BRIDON International Ltd. (DYFORM is a registered trademark of BRIDON PLC COMPANY.) The cable 60a includes nine outer strands 66, nine intermediate strands 64, and one inner strand 62. The outer strands 66 are indented to provide a smooth external periphery to reduce pressure leakage when running the cable 60a. Other than the 1×19 arrangement shown, the cable 60a can also come in a 1×16 conventional arrangement.
FIG. 3B illustrates a cross-sectional view of a mono-core cased hole cable 60b. The cable 60b has armor wires 64 disposed about an inner core wire 62. Typical sizes for the cable 60b are from 3/16 to ½-in. The armor wires 64 are the load bearing components of the cable 60b, and the inner core wire 62 is the conductor. The inner core wire 62 as the conductor is composed of copper wire and has shielding 65 around it. The armor wires 64 as the load bearing members are typically composed of steel and are not used as conductors. In some instances, a plastic sheath (not shown) may be disposed around the outside of the armor wires 64, but this is not strictly necessary.
Other forms of downhole running cables, such as slickline cables, used in the oilfield industry typically have metallic tubes that hold insulated copper conductors. The metallic tubes are typically made of Iconel® or other non-corrosive material. (INCONEL is a registered trademark of HUNTINGTON ALLOYS CORPORATION.) In many cases, the metallic tubes lack strength, and this prevents the slickline cables from being used with much pull force. Additionally, the slickline cables having the metallic tubes may need to pass through relatively small sheaves (16 to 20 in. in diameter) so the slickline cables may be prone to yielding and failure as they pass over the sheaves.
FIG. 3C illustrates a cross-sectional view of a conventional slickline cable 60c according to the prior art as described in U.S. Pat. No. 8,000,572. The cable 60c has a smooth outer surface so it can be used with a stuffing box and does not need a grease injection system. In cases where the cable 60c needs to effectively conduct electrical signals, the cable 60a typically has a copper wire core 72 for the cable's electrical conductors. The core 72 may instead have optical fiber conductors. Either way, the core 72 of copper wire or optical fiber lacks sufficient strength to carry tensile loads to which the cable 60c is subjected. Therefore, an outer metal tube 70 is used as the load-bearing member for the cable 60c and can surround the electrically conductive core 72 and any insulating layers.
For example, one such conductive slickline cable 60c has a solid copper wire core 72, an insulating jacket 74, and a serve of copper wires 76 on the outer diameter of the insulating jacket 74. A 316L stainless steel tube 70 is formed, welded, and drawn over the core 72, insulating jacket 74, and serve wires 76 to form a snug fit. The insulating jacket 74 can be composed of TEFLON (polytetrafluoroethylene and perfluoroalkox polymers and a trademark of E. I. du Pont de Nemours and Company of Wilmington, Del., U.S.A.). The tensile strength and fatigue life for this cable 60c are governed by the stainless steel tube 70 because the copper core 72 adds little strength.
In another arrangement, the slickline 60c uses an epoxy/fiber composite 76 sandwiched between two steel tubes 70 and 74 with optical fibers or copper conductors 72 contained in the inside tube 74. As shown in FIG. 3C, the fibers or conductors 72 are placed in the central metallic tube 74, which can be composed of stainless steel. Epoxy/long fiber composite 76 is then pultruded over the tube 74, and an outer tube 70 is generally placed over the composite 76. In this construction, the composite 76 provides a lightweight strength as well as a hydrogen-resistant barrier.
FIG. 3D illustrates a cross-sectional view of a coated slickline 60d according to the prior art. The slickline 60d has a standard slickline core 73, composed of stainless steel, surrounded by a polymer coating 77. One particular example of this type of slickline 60d is LIVE digital slickline available from Schlumberger Limited. The digital slickline 60d has an integral coating for digital two-way communication. A standard slickline unit and pressure control equipment deploy the slickline, and sensors downhole measure tension, detect shock and well deviation, and monitor temperature.
FIG. 3E illustrates a cross-sectional view of another prior art conductive slickline 60e for oilfield use, as described in U.S. Pat. No. 6,960,724. The cable 60e has a solid copper core conductor 72, a surrounding electrically insulating layer 74, and a tubular outer cover 76, which can be formed of a metal alloy. The core conductor 72 is electrically conductive, but lacks sufficient tensile strength to serve as a stress member for the cable 60e. Therefore, the outer cover 76 serves as the only stress member.
In some cables, the stress member of the outer cover 76 can be a solid component, such as a wire, rod, or tube. In other cables, the stress member of the outer cover 76 are formed of helically served wires, which are typically wrapped in two layers at similar angles, but in opposite directions. Together, the layers of wrapped wires serve as the stress member 76. The cable stress member 76 may also be braided, and may be fabricated from synthetic fibers, such as Kevlar (trademark of E. I. du Pont de Nemours and Company of Wilmington, Del., U.S.A.) or polyester.
In FIG. 3F, an electrical cable 60f such as disclosed in U.S. Pat. No. 6,960,724 has a solid core conductor 72, a surrounding electrically insulating layer 74, and a conductive tubular metal outer cover 76. Here, the core conductor 72 is a steel wire so that it is electrically conductive and has sufficient tensile strength to serve as an additional stress member for the cable 60f. The core conductor 72 and the outer cover 76 may, alternatively, be of braided wire construction. Thus, the cable 60f can have dual stress members, including the core conductor 72 and the outer cover 76, which are both electrically conductive.
To enhance its electrical conductivity, the core conductor 72 may be coated in copper or other highly electrically conductive material. Alternatively, a serve of copper wires 73 or copper tape may be applied to the surface of the core conductor 72 to increase its conductivity. The core conductor 72 may also be constructed of other electrically conductive materials that have the requisite tensile strength to act as a stress member, such as, aluminum or titanium, and, if of braided wire construction, may include a limited number of low tensile strength wire conductors, such as brass and copper. In yet a further alternative embodiment, the load-bearing core 72 may be constructed of a non-conductive carbon, glass, or synthetic fiber-reinforced plastic, with core conductivity provided by a copper or other highly conductive coating thereon.
The tubular metal outer cover 76 forms the second stress member of the cable 60f and also serves as the electrical return path. The outer cover 76 may be formed of any metal having suitable tensile strength and electrical conductivity, such as, for example, Inconel, stainless steel, galvanized steel, or titanium.
The dual stress members/conductors 72 and 76 are separated by electrically insulating layer 74, which is formed of a non-conductive material, such as TEFLON (polytetrafluoroethylene and perfluoroalkoxy polymers) or polyetheretherketone (PEEK). To enhance the electrical conductivity of the current path formed by the outer cover 76, the outer surface of the insulating layer 74 may be covered in a conductive material. This conductive material may be in the form of a coating, such as thermally sprayed copper, a conductive tape, or helically served wires 75.
In another prior art configuration, FIG. 3G shows a slickline cable 60g having of a solid wire core 72, an inner coating or jacket 74, and an outer coating or jacket 76. The outer coating 76 is resistant to abrasions and is smooth to allow easy travel through a packing assembly and blow-out preventer of the rig up assembly. The inner coating 74 can be heat resistant, and one or both of the coatings 74, 76 can be good insulators.
The outer coating 76 can be an epoxy, and the inner coating 74 can be a polyolephine. The outer coating 76 can be similar to the coating that is typically used on transformer windings with enhanced heat resistance and smoothness.
As shown in FIG. 3H, a slickline cable 60h of the prior art can include a conductive shield 78 between the inner coating 74 and the other coating 76. The conductive shield 78 acts as the return path.
As shown in FIG. 3I, the slickline cable 60i includes a hard jacketed cable. The hard jacketed cable 60i can include three parts of an outer tube 76, an insulating layer 74, and one or more conductors 72. The outer tube 76 is made of steel, such as a stainless steel similar to those used in a standard slickline cable. The thickness of the outer tube 76 is selected (a) to provide the strength necessary to pull and hold a tool and the cable 60i itself over the entire distance and depth that the tool is expected to operate in a borehole and/or (b) to be flexible enough to maneuver through the borehole or at least that portion of the borehole to be surveyed by the slickline tool.
The insulating layer 74 is a high temperature insulator that helps maintain the form of the outer tube 76. For example, the insulating layer 74 can include a magnesium oxide. The one or more conductors 72 can be copper wire and can be solid or stranded wire. The outer tube 76 acts as the return path and one or more of the conductors 72 acts as the forward path, or the reverse can be used. Alternatively, the conductors 72 can be used to provide power to a downhole tool.
As noted above, the cables can come in a variety of materials. The cable, such as the strands in FIG. 3A, can be composed of galvanized steel or 316 stainless steel depending of the well environment. Wirelines can be composed of plain carbon steel (API 9A), UHT carbon steel, and 316 stainless steel. Conductors for the cables can be composed of copper.
During use, the cables are subject to elastic elongation, permanent stretch, breakage, and the like based on the loads, twists, bends, and other actions subjected to the cable. Another source of stretch to the cable comes from elastic extension of the cable under load, which is typically characterized as linear in nature. Permanent elongation can occur when high loads on the cable produce uniform plastic yielding. Additionally, localized plastic yielding may occur after a maximum breaking load is exceeded. When the cable is moved in the well, frictional forces also act on the cable and can add to the line tension especially during recovery.
Many cables have helically wound lines that generate torque when under axial load. The cables therefore tend to unlay or untwist to some extent under certain circumstances. Factors surrounding this behavior can be very difficult to predict. Even thermal expansion can occur during use of the cable, although thermal effects may not alter the mechanical properties of the cable's composition.
With all of the forms of elongation, twisting, plastic deformation, etc. that a cable can encounter, the service history of the cable needs to monitored and logged to determine what loads and actions the cable has been subjected to so an assessment can be made whether the cable is still serviceable or not. Additionally, operators need to monitor and tabulate the length of the cable to know where tools are actually located in the well and to perform various operations downhole with the cable.
Although there are many types of downhole cables known in the art and even though they may be effective, operators are continually increasing other types of uses for downhole cables and subjecting the cables to ever changing conditions and environments. To that end, the subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.